White blood cells called T lymphocytes play critical roles in immune defense against viruses, bacteria, fungi, protozoa, and cancer cells. They are also involved in allergies/asthma and in autoimmune diseases. The effector functions of T cells are mediated largely by proteins termed cytokines that either be expressed at the cell surface or secreted. T cells see foreign substances (antigens) in the form of peptide-major histocompatibility complex (MHC) molecule complexes on cell surfaces. We wish to know how such complexes interact with specific receptors to evoke the effector activities of mature T cells in the body, as well as regulate their growth, inactivation, or death. A central issue is the relationship between the amount of signaling received by the cell from receptors seeing peptide-MHC ligand and the functional response of each T cell. Another major concern is the extent to which different cells, even with identical receptors, show different biological response patterns. We have used new methods for analyzing T cell recognition events and cytokine production by individual cells to address these issues. We previously showed that both T cells exhibit a hierarchical organization of response thresholds - in other words, more antigen is needed to elicit one particular response as compared to another from the same T cell. Therefore, as antigen concentration changes, so does the mix of effector molecules produced by the cells. This alters the quality as well as the quantity of an immune response as the amount of antigen changes in a host. Our prior studies also revealed cell to-cell heterogeneity in the responses of T cells with identical antigen receptors. During this past year we have examined whether the nature of TCR signaling changes as T cells adopt polarized ?Th1? vs. ?Th2? phenotypes and found that indeed early tyrosine phosphorylation events are distinct in T cells with identical receptors but producing different cytokines. Preliminary data suggest this is at least in part related to an alteration in expression of key surface proteins involved in the antigen recognition process.